1. Field of the Invention
The present invention relates to a novel superalloy used for blades (or buckets) and nozzles (vanes) of a gas turbine for aircraft or for land base which requires high creep rupture strength in the combustion gas atmosphere at a high temperature, a single crystal casting produced by utilizing such a superalloy, single crystal structure members for a gas turbine produced by utilizing such a casting having single crystal structure, and a high efficient gas turbine produced by utilizing such a single crystal structure member and, more particularly, for such uses with respect to land-based gas turbine which requires a material having high hot corrosion resistance. This invention also relates to a high efficient combined cycle power generation system produced by utilizing such a high efficient gas turbine.
2. Description of the Related Art
In response to the rise in the combustion temperature corresponding to high power and high efficiency of a gas turbine, the alloy used for a turbine blade which is exposed to the harshest use has been shifted from a conventional casting alloy having a polycrystal structure to an alloy having a directional columnar crystal structure which has no crystal grain boundary in the direction in which stress is applied, and further to a single crystal alloy which has no grain boundary whatsoever. Such a development has been mainly seen in a gas turbine for aircraft which has a rather short continuous operation time, and high creep rupture strength and high fatigue strength to low cyclic variation of load have been considered -to be important.
Alloy 444 (U.S. Pat. No. 4,116,723, JP-B2-59-34776), PWA 1484 (U.S. Pat. No. 4,719,080, JP-A-61-284545, "Second-generation Nickel-base Single Crystal Superalloy"; A. D. Cetel and D. N. Duhl; Superalloys 1988, The Metall. Soc., (1988), pp. 235-244), CMSX-4 (U.S. Pat. No. 4,643,782, JP-A-60-211031, "Process and Alloy Optimization for CMSX-4 Superalloy Single Crystal Airfoils"; D. J. Fraiser, J. R. Whetstone, K. Harris, G. L. Erickson, R. E. Schwer; High Temp. Mater. Power Eng. 1990 Part 2, (1990), pp. 1281-1300), and SC-83K (U.S. Pat. No. 4,976,791, JP-A-2138431, "Development of Nickel-base Single Crystal Superalloy"; Takehiro Ohno and Rikizo Watanabe; "Iron and Steel", vol. 77, (1991) pp. 832-839) have been developed used for the purpose of a blade of a gas turbine for aircraft, and they do not necessarily have sufficient high hot corrosion resistance.
At present, the members of a gas turbine, such as blades and nozzles, are protected from the harshly corrosive environment by spray coating with alloy having high corrosion resistance and also spray coating with ceramics on the surface of the members of a gas turbine in low pressure plasma atmosphere. However, the coating on the surface is occasionally peeled off, and it is desirable that the hot corrosion of the peeled-off portions be extremely delayed from progressing as low as possible. Since a gas turbine for aircraft has a short continuous operation time and a short inspection cycle, it can be still used even though it has a single crystal superalloy with insufficient corrosion resistance. Nevertheless, it is still desirable to reduce the maintenance work which results in improving reliability. Thus the improvement in hot corrosion resistance of a superalloy is in great demand.
On the other hand, a conventional casting alloy is usually used for blades of a gas turbine of land-base use because it is used at a lower temperature than that of a gas turbine for aircraft. However, due to the recent global environmental pollution issues, a high-efficient land-based gas turbine is rapidly coming into demand, and the combustion temperature of a gas turbine is rising even for land-base use. Accordingly, an alloy for the blade needs to shift from an ordinary casting alloy to a single crystal alloy. A gas turbine of land-base use has a much longer continuous operation time than that of aircraft, and accordingly, it has a long interval time between inspections. Therefore, it is particularly important to delay the progressing of hot corrosion when the coating of a blade of a gas turbine has peeled off. Thus, it is necessary to provide higher hot corrosion and oxidation resistance under the combustion gas atmosphere for a land-based gas turbine than for a gas turbine for aircraft. However, a single crystal alloy used for a gas turbine which possesses such high hot corrosion resistance has not yet been put to practical use.
Also, the most significant factor for facilitating efficiency of a gas turbine is to raise the temperature under which first stage blades and nozzles can endure. If a highly hot corrosion resistant and high-strength superalloy and a single crystal casting structure using such a superalloy which are able to solve the above-mentioned problem are produced, the combustion temperature of a gas turbine can be raised. As a result, the efficiency of a gas turbine is facilitated.
Furthermore, if such a high-efficiency gas turbine is produced, it is possible to obtain a high-efficient combined cycle power generation system having more than 50% of heat efficiency of the whole plant, which system cannot be obtained according to the related art, in a complex plant having an exhaust heat recovery boiler which obtains steam from the energy of the gas exhausted by such a gas turbine, a steam turbine driven by the steam, and a power generator driven by the gas turbine and the steam turbine.
Sodium chloride and sodium sulfate are generally known as corrosion reactants contained in the combustion gas of a gas turbine of land-base use. In general, corrosion resistance is evaluated by various tests using such salt mixture, such as a burner rig method, a coating and heating method, and a total immersion in molten salt method. In the case where a single crystal alloy used in a gas turbine of land-base use is employed, it is required to have hot corrosion resistance as substantially the same level, or close to the level of that of Rene 80 (60Ni-14Cr-9.5Co-4Mo-4W-3A1-5Ti-0.17C-0.015B-0.03Zr) which has been widely used as a conventional casting alloy. Moreover, in the case where a single crystal alloy is exposed under a higher temperature than Rene 80, it is also required to have even more excellent oxidation resistance than Rene 80.
The present inventors have compared corrosion resistance of various kinds of conventional alloys for single crystal casting and that of Rene 80 by employing a test in which the alloys are immersed totally in a molten salt as a means of evaluating their corrosion resistance, as shown in an example which will be described later. In this test, these alloys have been tested under polycrystal state. Among the above-mentioned single crystal alloys (Alloy 444 has not been evaluated since it does not have the strength at which the present invention aims.), PWA1484 and SC-83K are known as alloys having the highest creep rupture strength in the world. However, in the evaluation of the present experiment, a piece of these alloys being tested for hot corrosion resistance has been completely melted in the crucible. Thus, the present inventors have discovered that these alloys do not have sufficient hot corrosion resistance for blades of a land-based gas turbine.
On the other hand, the corrosion resistance of CMSX-4 having a lower high temperature strength than PWA1484 and SC-83K has completely fallen short of the hot corrosion resistance of Rene 80 though it is more excellent in hot corrosion resistance than conventional single crystal alloys.